1. Field of the Disclosure
The disclosure relates generally to downhole motors used to drill boreholes in earthen formations for the ultimate recovery of oil, gas, or minerals. More particularly, the disclosure relates to downhole motors including adjustable bend assemblies for directional drilling.
2. Background of the Technology
In drilling a borehole into an earthen formation, such as for the recovery of hydrocarbons or minerals from a subsurface formation, it is conventional practice to connect a drill bit onto the lower end of a drillstring formed from a plurality of pipe joints connected together end-to-end, and then rotate the drill string so that the drill bit progresses downward into the earth to create a borehole along a predetermined trajectory. In addition to pipe joints, the drillstring typically includes heavier tubular members known as drill collars positioned between the pipe joints and the drill bit. The drill collars increase the vertical load applied to the drill bit to enhance its operational effectiveness. Other accessories commonly incorporated into drill strings include stabilizers to assist in maintaining the desired direction of the drilled borehole, and reamers to ensure that the drilled borehole is maintained at a desired gauge (i.e., diameter). In vertical drilling operations, the drillstring and drill bit are typically rotated from the surface with a top dive or rotary table.
During the drilling operations, drilling fluid or mud is pumped under pressure down the drill string, out the face of the drill bit into the borehole, and then up the annulus between the drill string and the borehole sidewall to the surface. The drilling fluid, which may be water-based or oil-based, is typically viscous to enhance its ability to carry borehole cuttings to the surface. The drilling fluid can perform various other valuable functions, including enhancement of drill bit performance (e.g., by ejection of fluid under pressure through ports in the drill bit, creating mud jets that blast into and weaken the underlying formation in advance of the drill bit), drill bit cooling, and formation of a protective cake on the borehole wall (to stabilize and seal the borehole wall).
Recently, it has become increasingly common and desirable in the oil and gas industry to drill horizontal and other non-vertical or deviated boreholes (i.e., “directional drilling”), to facilitate greater exposure to and production from larger regions of subsurface hydrocarbon-bearing formations than would be possible using only vertical boreholes. In directional drilling, specialized drill string components and “bottomhole assemblies” (BHAs) are often used to induce, monitor, and control deviations in the path of the drill bit, so as to produce a borehole of the desired deviated configuration.
Directional drilling is typically carried out using a downhole or mud motor provided in the bottomhole assembly (BHA) at the lower end of the drillstring immediately above the drill bit. Downhole motors typically include several components, such as, for example (in order, starting from the top of the motor): (1) a power section including a stator and a rotor rotatably disposed in the stator; (2) a drive shaft assembly including a drive shaft disposed within a housing, with the upper end of the drive shaft being coupled to the lower end of the rotor; and (3) a bearing assembly positioned between the driveshaft assembly and the drill bit for supporting radial and thrust loads. For directional drilling, the motor often includes a bent housing to provide an angle of deflection between the drill bit and the BHA. The deflection angle is usually between 0° and 5°. The axial distance between the lower end of the drill bit and bend in the motor is commonly referred to as the “bit-to-bend” distance.
To drill straight sections of borehole with a bent motor, the entire drillstring and BHA are rotated from the surface with the drillstring, thereby rotating the drill bit about the longitudinal axis of the drillstring; and to change the trajectory of the borehole, the drill bit is rotated exclusively with the downhole motor, thereby enabling the drill bit to rotate about its own central axis, which is oriented at the deflection angle relative to the drillstring due to the bent housing. Since the drill bit is skewed (i.e., oriented at the deflection angle) when the entire drillstring is rotated while drilling straight sections, the downhole motor is subjected to bending moments which may result in potentially damaging stresses at critical locations within the motor.